The invention relates to an oxygen-consuming electrode, more particularly for use in chloralkali electrolysis, comprising a novel catalyst coating based on silver and a finely divided silver oxide, and to an electrolysis apparatus. The invention further relates to a production process for the oxygen-consuming electrode and the use thereof in chloralkali electrolysis or fuel cell technology.
The invention proceeds from oxygen-consuming electrodes known per se, which take the form of gas diffusion electrodes and typically comprise an electrically conductive carrier and a gas diffusion layer comprising a catalytically active component.
Various proposals for production and operation of the oxygen-consuming electrodes in electrolysis cells on the industrial scale are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode in the electrolysis (for example in chloralkali electrolysis) with the oxygen-consuming electrode (cathode). An overview of the possible cell designs and solutions can be found in the publication by Moussallem et al “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-consuming electrode—also called OCE for short hereinafter—has to meet a series of requirements to be usable in industrial electrolysers. For instance, the catalyst and all other materials used have to be chemically stable towards approx. 32% by weight sodium hydroxide solution and towards pure oxygen at a temperature of typically 80-90° C. Similarly, a high degree of mechanical stability is required, since the electrodes are installed and operated in electrolysers with a size typically more than 2 m2 in area (industrial scale). Further properties are: high electrical conductivity, low layer thickness, high internal surface area and high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and a corresponding pore structure for conduction of gas and electrolyte are likewise necessary, as is such imperviosity that gas and liquid space remain separate from one another. Long-term stability and low production costs are further particular requirements on an industrially usable oxygen-consuming electrode.
An oxygen-consuming electrode consists typically of a carrier element, for example a plate of porous metal or metal wire mesh, and an electrochemically active coating. The electrochemically active coating is microporous and consists of hydrophilic and hydrophobic constituents. The hydrophobic constituents make it difficult for electrolytes to penetrate and thus keep the corresponding pores unblocked for the transport of the oxygen to the catalytically active sites. The hydrophilic constituents enable the electrolyte to penetrate to the catalytically active sites, and the hydroxide ions to be transported away. The hydrophobic component used is generally a fluorinated polymer such as polytetrafluoroethylene (PTFE), which additionally serves as a polymeric binder of the catalyst. In the case of electrodes with a silver catalyst, the silver serves as a hydrophilic component. In the case of carbon-supported catalysts, the carrier used is a carbon with hydrophilic pores, through which liquid can be transported.
The oxygen is reduced in a three-phase region, in which gas phase, liquid phase and solid catalyst are in contact.
The gas is transported through the pores in the hydrophobic matrix. The hydrophilic pores fill up with liquid; the water is transported to the catalytic sites and the hydroxide ions away from them through these pores. Since oxygen dissolves in the aqueous phase only to a limited degree, sufficient water-free pores must be available for transport of the oxygen.
A multitude of compounds have been described as catalysts for the reduction of oxygen.
For instance, there are reports of the use of palladium, ruthenium, gold, nickel, oxides and sulphides of transition metals, metal porphyrins and phthalocyanines, and perovskites as catalysts for oxygen-consuming electrodes.
However, only platinum and silver have gained practical significance as catalysts for the reduction of oxygen in alkaline solutions.
Platinum has a very high catalytic activity for the reduction of oxygen Due to the high costs of platinum, it is used exclusively in supported form. A known and proven support material is carbon. Carbon conducts electrical current to the platinum catalyst. The pores in the carbon particles can be hydrophilized by oxidation of the carbon surface, and become suitable for the transport of water as a result. However, the stability of carbon-supported platinum electrodes in long-term operation is inadequate, probably because platinum also catalyses the oxidation of the support material. The oxidation of the support material leads to loss of the mechanical stability of the electrode.
Silver likewise has a high catalytic activity for the reduction of oxygen
According to the prior art, silver can also be used with carbon as a support, and also in the form of finely divided metallic silver.
OCEs comprising carbon-supported silver typically have silver concentrations of 20-50 g/m2. Even though the carbon-supported silver catalysts are fundamentally more durable than the corresponding platinum catalysts, long-term stability under the conditions of chloralkali electrolysis is limited.
It is an object of the present invention to provide an oxygen-consuming electrode, more particularly for use in chloralkali electrolysis, in which silver oxide is used, which enables a lower operating voltage in chloralkali electrolysis, and the production process for which overcomes the aforementioned disadvantages.
It has been found that, surprisingly, the use of a silver oxide as a catalytically active material in gas diffusion electrodes, said material having been produced by the following steps:                (1) precipitating silver oxide in an aqueous NaOH solution, (This involves using a suitable stirrer to introduce a defined stirring energy, especially a propeller stirrer, with a stirrer speed of 300-1000 rpm, keeping the pH constant within the range from pH 10 to 12, preferably at pH 11, and keeping the temperature within a range from 20° C. to 80° C., preferably 30° C. to 70° C.)        (2) filtering and washing the filtercake (This step is optionally repeated twice or more. After the last wash, the suspension is filtered once again.),        (3) drying the filtercake under inert gas, e.g. nitrogen or noble gas, in a drying cabinet, optionally under reduced pressure (5 to 1000 mbar), at a temperature in the range from 80° C. to 200° C., and further processing the silver oxide formed by selected production processes to give an OCE,leads to lower cell voltages in chloralkali electrolysis.        